1. Field of the Invention:
The invention relates to a process for improving the color quality of molding compounds that contain polyphenylene ether (PPE) or consist of it exclusively.
2. Discussion of the Background:
It is known that PPE is very sensitive to thermal stress, which manifests itself in the formation of dark-colored products with a quinone structure, even in the absence of oxygen. Impurities of all types naturally affect substantially the color quality of PPE and molding compounds containing PPE. The problem becomes particularly important when the PPE is not isolated by precipitation with an antisolvent, but a so-called direct isolation process is followed, which is more beneficial from the process point of view. Such a process consists of removing interfering impurities, such as catalyst components and byproducts of the reaction such as diphenoquinone, from the polymer formed after the oxidative coupling reaction in solution, elimination of most of the solvent by distillation, and introduction of the material into an extruder, optionally with the addition of other polymer components, flameproofing agents, and other additives.
It is known that the terminal OH groups of polyphenylene ether can be converted into a stable form by reaction with reactive compounds such as acid chlorides and acid anhydrides, for example.
JP-OS No. 71/02837 discloses reacting the terminal groups of PPE with acrylamides in the presence of basic compounds to protect the PPE against oxidative degradation. Nevertheless, unsatisfactory color changes also occur in such masked polyphenylene ethers with the evolution of heat. It must therefore be assumed that the observed reactions are not restricted to the terminal groups of PPE.
German Pat. No. 16 94 258 discloses the addition of 0.1 to 6 wt. % of organic phosphites as stabilizers for PPE. Such stabilizers prevent oxidative damage of the PPE in the further processing of a powder or granulate.
It has also been proposed to add chelating agents and other stabilizers such as difunctional phenols and reducing agents (see DE-OS No. 27 54 887 and DE-PS No. 26 16 746) or aromatic amines (see DE-OS No. 27 55 937) to the polymer solutions. While the chelating agents are said to eliminate the harmful effects of residues of the metal catalyst in the product, the reducing agents serve to convert colored quinones. These solutions are unsatisfactory since the additives are sometimes needed in substantial concentrations. They also make the isolation of the polyphenylene ethers more difficult. In particular, no color stabilization is achievable by these measures when using higher temperatures.
Another added difficulty is the fact that PPE alone and its mixtures with styrene polymers tend to increase in molecular weight during the extrusion. This is undesirable since the flow characteristics of the polymer then deteriorate and higher temperatures become necessary in the extrusion (see EP-OS No. 0 121 974, page 1). To avoid this, it is proposed to stabilize the molecular weight during the extrusion of PPE or PPE molding compounds by adding solid dienophiles such as maleimides, or maleio acidamides whIoh are oonverted into maleimides under the described reaction conditions, and acenaphthene. These compounds are thermally unstable, i.e., thermal polymerization occurs at higher temperatures. Bismaleimides, which develop crosslinked structures even between 180.degree. and 240.degree. C. (see Polym. 26, 1561 ff. (1985)) are expressly excluded (see U.S. Pat No. 4,389,516=DE-PS No. 31 11 403). This "curing" is said to occur through free radicals as shown below. ##STR2##
If crosslinked composites are obtained in this reaction in the prsence of PPE, as stated in EP-OS No. 0 121 974, page 3, lines 1-5, then it must be assumed that the PPE in the case of the bismaleimides is incorporated in this network by radical transfer reactions. This is undesirable since it amounts to an increase of the molecular weight with the undesirable consequences noted above.
According to information disclosed in DE-OS No. 24 30 130, the oxidative coupling reaction of monomeric phenols proceeds even after termination of the actual reaction, with a "zipper-like depolymerization reaction" occurring on the one hand and a crosslinking reaction on the other. The latter is caused by the fact that hydrogen atoms on the alpha-carbon atom of the side chain are split off and active methylene groups are formed. These reactions take place even when the catalyst is deactivated. After treatment of the PPE simultaneously with a difunctional phenol or benzoquinone and a mild reducing agent such as hydrazine (see DE-PS No. 24 30 130), it is in fact possible to obtain a PPE of reproducible molecular weight after the oxidative coupling reaction. However, the problems that occur during the extrusion are not solved.
Graft polymers prepared by polymerization of styrene in the presence of PPE and a radical initiator, are said to have an improved color quality according to JP-OS No. 72/105,414. This process is thus unsuitable for molding compounds that contain no polystyrene.